In optical communication systems, a light signal is encoded with a binary stream of data at a transmitter location and is sent towards a receiver location where it can be decoded, so as to re-create the stream of data at the receiver location. The most direct method of encoding of a light signal is to set an optical power level of the signal in dependence upon the information bit being transmitted. For example, a light source can be turned on when a “one” is being transmitted, and turned off when a “zero” is being transmitted. Alternatively, an optical shutter may be opened or closed. Due to its simplicity, the optical power level modulation, commonly known as on-off keying (OOK), is most frequently used in modern fiberoptic and, or free-space optical communication systems.
The data can also be encoded into a phase shift of a signal. This method is commonly called “phase shift keying” (PSK) and is extensively used in radio frequency communications and broadcasting. In a PSK method, the signal to be decoded is mixed in a coherent mixer device with a local reference signal from a “local oscillator” (LO), which serves as a phase reference, so as to make the two signals interfere coherently with each other. By using the PSK encoding and decoding method, the performance of an optical transmission link can be greatly enhanced as compared to the OOK method, due to increased sensitivity and selectivity of detection, which allows one to increase both overall transmission distance and amount of the data transmitted over a single communication link.
Despite all the advantages and a massive research effort undertaken in the 1980s and 1990s, a PSK encoding and a coherent detection methods have not yet found a widespread application in modern optical communication systems, mostly due to technical difficulties associated with their practical implementation. The difficulties arise from complexity of transmitters and receivers and reliability concerns of data transmission using the PSK method, as compared to the quite reliable and well-established OOK method. Due to the coherent nature of optical interaction of a signal with a reference and resulting sensitivity of the mixed signal to slight fluctuations of the optical path length differences in a coherent mixer, the requirements on mechanical and thermal stability of a coherent receiver are higher as compared to the requirements on stability of a traditional receiver employing the OOK method. Not only that, but the requirements on stability of a LO are very stringent, and an active control of the latter is frequently implemented. Furthermore, the manufacturing tolerances of the mixer optics, which comprise an optical interferometer, are quite tight. The tight tolerances raise manufacturing costs due to reduced manufacturing yields.
One of important challenges of coherent detection is related to instability of polarization of an optical signal being detected. A light wave is a transversal wave; therefore, to make it interfere with a reference light wave to obtain the phase information, the polarization states of the two waves have to be matched. In this regard, it should be noted that a vast majority of fiberoptic networks are implemented using a conventional single-mode optical fiber that does not maintain optical polarization of light passing therethrough, since a polarization state of an optical signal is irrelevant in the OOK detection method, which has already been in practical use when large amounts of the conventional fiber were installed worldwide. Consequently, a polarization state of optical signals used in modern fiberoptic networks is random, and is varying in time. As a result, the amplitude of a signal at the output of a single-polarization coherent mixer varies in time, too, introducing severe fading and even complete signal loss under certain conditions.
A polarization controller may be used to stabilize the polarization state of an incoming optical signal. However, polarization controllers are expensive and complicated devices, and their installation for every optical channel is usually cost-prohibitive. An elegant solution to the polarization instability problem, known in the art as “polarization diversity”, is to split the incoming optical signal into two sub-signals having orthogonal states of polarization, and to mix the two sib-signals separately with two reference sub-signals conveniently split from a common reference signal. For example, in U.S. Pat. No. 5,307,197 by Tanabe et al., which is incorporated herein by reference, an optical circuit for a polarization diversity receiver is described, comprising a waveguide-based polarization splitter for splitting the signal into two portions, two optical waveguide couplers for mixing these two portions with a reference signal, and two waveguide-based polarization combiners for combining the signal back into the polarization-insensitive form, to be detected by a double-balanced receiver. Disadvantageously, the waveguide-based polarization splitters and combiners are formed on a LiNbO3 crystal substrate, which rises the manufacturing costs of the device as compared to waveguides implemented on a more widely used silicon substrate.
A coherent optical detector employing polarization diversity is described in U.S. Pat. No. 7,280,770 by Tan et al., which is incorporated herein by reference. The coherent optical detector of Tan et al. combines a single polarization-maintaining waveguide coupler, used for both polarization components of an incoming signal, and a free-space polarization diversity arrangement including a polarizer, a walk-off crystal tilted at 45 degrees, a GRIN lens, and a two-dimensional fiber array for holding output optical fibers. While the detector of Tan et al. only uses one coupler, which is advantageous from the material cost standpoint, one disadvantage of the approach of Tan et al. is the complexity of alignment resulting from a requirement to match the pitch of the two-dimensional fiber array for four output fibers to an output light spot pattern, which depends upon thickness of the walk-off birefringent crystal.
A coherent optical detection apparatus that does not require a polarization diversity arrangement for coherent detection of a randomly-polarized optical signal is described in U.S. Pat. No. 7,327,913 by Shpantzer et al., which is incorporated herein by reference. The coherent optical detector of Shpantzer et al. comprises a four-coupler optical mixer on an electro-optic substrate such as LiNbO3 crystal substrate, wherein all four couplers and at least two phase delay elements, disposed between the couplers, are electrically controlled. A digital signal processor (DSP) is utilized to extract the signal polarization information from two balanced pairs of optical signals at the output of the four-coupler optical mixer chip. The local oscillator polarization and, or the local oscillator phase is adjusted to compensate for thermal drifts and optimize the efficiency of coherent detection. Disadvantageously, the apparatus of Shpantzer et al. requires a complicated fast DSP control circuitry for controlling the coupling ratios of the couplers, the phase delays of the tunable phase delay elements, and polarization of the local oscillator, which raises the cost and complexity of the coherent detector apparatus.
Despite a great variety of devices and methods for coherent detection of light, the practical implementation of coherent transmitters and detectors in optical communication systems has been hindered by such factors as complexity, high cost, environmental instability, and insufficient reliability of data transmission. It is, therefore, an objective of the present invention to provide an inexpensive, low-cost, environmentally stable coherent optical mixer suitable for a large-scale deployment over the existing fiberoptic network infrastructure employing a single-mode non-polarization-maintaining (non-PM) optical fiber. It is further objective of the present invention to provide a reliable and inexpensive method of coherent detection of an optical signal transmitted over a span of a non-PM fiber.